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Chapter 14 Mendel and the
Gene Idea
Genetics
The scientific study of inheritance.
Genetics is a relatively “new” science (about 150 years).
Genetic Theories1. Blending Theory -
traits were like paints and mixed evenly from both parents.
2. Incubation Theory -
only one parent controlled the traits of the children.Ex: Spermists and Ovists
3. Particulate Model -
parents pass on traits as discrete units that retain their identities in the offspring.
Gregor Mendel Father of Modern Genetics.
Mendel was a pea picker.
He used peas as his study organism.
Why Use Peas?
Short life span. Bisexual. Many traits known. Cross- and self-pollinating. (You can eat the failures).
Cross-pollination
Two parents. Results in hybrid offspring
where the offspring may be different than the parents.
Self-pollination
One flower as both parents. Natural event in peas. Results in pure-bred
offspring where the offspring are identical to the parents.
Mendel's Work
Used seven characters, each with two expressions or traits.
Example: Character - height
Traits - tall or short.
Monohybrid or Mendelian Crosses
Crosses that work with a single character at a time.
Example - Tall X short
P Generation
The Parental generation or the first two individuals used in a cross.
Example - Tall X short Mendel used reciprocal crosses,
where the parents alternated for the trait.
Offspring
F1 - first filial generation. F2 - second filial generation,
bred by crossing two F1 plants together or allowing a F1 to self-pollinate.
Results - Summary In all crosses, the F1
generation showed only one of the traits regardless of which was male or female.
The other trait reappeared in the F2 at ~25% (3:1 ratio).
Mendel's Hypothesis
1. Genes can have alternate versions called alleles.
2. Each offspring inherits two alleles, one from each parent.
Mendel's Hypothesis3. If the two alleles differ, the
dominant allele is expressed. The recessive allele remains hidden unless the dominant allele is absent.
Comment - do not use the terms “strongest” to describe the dominant allele.
Mendel's Hypothesis
4. The two alleles for each trait separate during gamete formation. This now called: Mendel's Law of Segregation
Law of Segregation
Mendel’s Experiments
Showed that the Particulate Model best fit the results.
Vocabulary
Phenotype - the physical appearance of the organism.
Genotype - the genetic makeup of the organism, usually shown in a code. T = tall t = short
Helpful Vocabulary
Homozygous - When the two alleles are the same (TT/tt).
Heterozygous- When the two alleles are different (Tt).
Test Cross
Cross of a suspected heterozygote with a homozygous recessive.
Ex: T_ X tt
If TT - all dominant
If Tt - 1 Dominant: 1 Recessive
Dihybrid Cross
Cross with two genetic traits. Need 4 letters to code for the
cross. Ex: TtRr
Each Gamete - Must get 1 letter for each trait. Ex. TR, Tr, etc.
Number of Kinds of Gametes
Critical to calculating the results of higher level crosses.
Look for the number of heterozygous traits.
Equation
The formula 2n can be used, where “n” = the number of heterozygous traits.
Ex: TtRr, n=2
22 or 4 different kinds of gametes are possible.
TR, tR, Tr, tr
Dihybrid Cross
TtRr X TtRr
Each parent can produce 4 types of gametes.
TR, Tr, tR, tr
Cross is a 4 X 4 with 16 possible offspring.
Results
9 Tall, Red flowered 3 Tall, white flowered 3 short, Red flowered 1 short, white flowered
Or: 9:3:3:1
Law of Independent Assortment
The inheritance of 1st genetic trait is NOT dependent on the inheritance of the 2nd trait.
Inheritance of height is independent of the inheritance of flower color.
Comment #1
Ratio of Tall to short is 3:1 Ratio of Red to white is 3:1 The cross is really a product
of the ratio of each trait multiplied together. (3:1) X (3:1)
Probability
Genetics is a specific application of the rules of probability.
Probability - the chance that an event will occur out of the total number of possible events.
Genetic Ratios
The monohybrid “ratios” are actually the “probabilities” of the results of random fertilization.
Ex: 3:175% chance of the dominant25% chance of the recessive
Rule of Multiplication
The probability that two alleles will come together at fertilization, is equal to the product of their separate probabilities.
Example: TtRr X TtRr
The probability of getting a tall offspring is ¾.
The probability of getting a red offspring is ¾.
The probability of getting a tall red offspring is ¾ x ¾ = 9/16
Comment
Use the Product Rule to calculate the results of complex crosses rather than work out the Punnett Squares.
Ex: TtrrGG X TtRrgg
Solution
“T’s” = Tt X Tt = 3:1 (Tall:Short)
“R’s” = rr X Rr = 1:1 (Red:White)
“G’s” = GG x gg = 1:0 (Yellow:green)
Product is:
(3:1) X (1:1) X (1:0 ) = 3:3:1:1
Tall, Red, Green peas (3x1x0)
Variations on Mendel
1. Incomplete Dominance
2. Codominance
3. Multiple Alleles
4. Sex-Linked
5. Polygenic Inheritance
Incomplete Dominance
When the F1 hybrids show a phenotype somewhere between the phenotypes of the two parents.
Ex. Red X White snapdragons F1 = all pink F2 = 1 red: 2 pink: 1 white
Result No hidden Recessive. 3 phenotypes and
3 genotypes Red = CR CR
Pink = CRCW
White = CWCW
Codominance
Both alleles are expressed equally in the phenotype.
Ex. Sickle Cell Anemia AA=Normal blood cells AA’=Some normal some sickle A’A’= All Sickle shaped
Result
No hidden Recessive. 3 phenotypes and
3 genotypes
Multiple Alleles
When there are more than 2 alleles for a trait.
Ex. ABO blood group IA - A type antigen IB - B type antigen i - no antigen
Result
Multiple genotypes and phenotypes.
Very common event in many traits.
Alleles and Blood Types
Type Genotypes
A IA IA or IAi B IB IB or IBi AB IAIB
O ii
Comment
Rh blood factor is a separate factor from the ABO blood group.
Rh+ = dominant Rh- = recessive A+ blood = dihybrid trait
Linked genes
There are many genes, but only a few chromosomes.
Therefore, each chromosome must carry a number of genes together as a “package”.
Linked Genes
Traits that are located on the same chromosome.
Result: Failure of Mendel's Law of
Independent Assortment. Ratios mimic monohybrid
crosses.
Crossing-Over
Breaks up linkages and creates new ones.
Recombinant offspring formed that doesn't match the parental types.
If Genes are Linked:
Independent Assortment of traits fails.
Linkage may be “strong” or “weak”.
Linkage Strength
Degree of strength related to how close the traits are on the chromosome. Weak - farther apart Strong - closer together
End of part 1
Chromosomal Basis of Sex in Humans
X chromosome - medium sized chromosome with a large number of traits.
Y chromosome - much smaller chromosome with only a few traits.
Human Chromosome Sex
Males - XYFemales - XX
Comment - The X and Y chromosomes are a homologous pair, but only for a small region at one tip.
Sex Linkage
Inheritance of traits on the sex chromosomes.
X- Linkage (common) Y- Linkage (very rare if exists
at all)
Males Hemizygous - 1 copy of X
chromosome. Show ALL X traits
(dominant or recessive). More likely to show X
recessive gene problems than females.
X-linked Disorders
Color blindness Duchenne's Muscular
Dystrophy Hemophilia (types a and b) Immune system defects
X-linked Patterns Trait is usually passed from a
carrier mother to 1/2 of sons. Affected father has no
affected children, but passes the trait on to all daughters who will be carriers for the trait.
Can Females be color-blind?
Yes, if their mother was a carrier and their father is affected.
Sex Limited Traits
Traits that are only expressed in one sex.
Ex – prostate
Sex Influenced Traits Traits whose expression
differs because of the hormones of the sex.
These are NOT on the sex chromosomes.
Ex. – beards, mammary gland development, baldness
Polygenic Inheritance
Factors that are expressed as continuous variation.
Lack clear boundaries between the phenotype classes.
Ex: skin color, height
Genetic Basis
Several genes govern the inheritance of the trait.
Ex: Skin color is likely controlled by at least 4 genes. Each dominant gives a darker skin.
Result Mendelian ratios fail. Traits tend to "run" in
families. Offspring often intermediate
between the parental types. Trait shows a “bell-curve” or
continuous variation.
Genetic Studies in Humans
Often done by Pedigree charts. Why?
Can’t do controlled breeding studies in humans.
Small number of offspring. Long life span.
Pedigree Chart Symbols
Male
Female
Person with trait
Sample Pedigree
Dominant Trait Recessive Trait
Human Recessive Disorders
Several thousand known: Albinism Sickle Cell Anemia Tay-Sachs Disease Cystic Fibrosis PKU Galactosemia
Sickle-cell Disease Most common inherited disease
among African-Americans. Single amino acid substitution
results in malformed hemoglobin. Reduced O2 carrying capacity. Codominant inheritance.
Recessive Pattern
Usually rare. Skips generations. Occurrence increases with
inbred matings.
Human Dominant Disorders
Less common then recessives.
Ex: Huntington’s disease Achondroplasia Familial Hypercholsterolemia
Inheritance Pattern
Each affected individual had one affected parent.
Doesn’t skip generations. Homozygous cases show
worse phenotype symptoms. May have post-maturity onset
of symptoms.
General FormalR = F X M X D
R = riskF = probability that the female
carries the gene.M = probability that the male
carries the gene.D = Disease risk under best
conditions.
Example
Wife has an albino parent. Husband has no albinism in
his pedigree. Risk for an albino child?
Risk Calculation Wife = probability is 1.0 that
she has the allele. Husband = with no family
record, probability is near 0. Disease = this is a recessive
trait, so risk is Aa X Aa = .25 R = 1 X 0 X .25 R = 0
Risk Calculation
Assume husband is a carrier, then the risk is:
R = 1 X 1 X .25
R = .25
There is a .25 chance that any child will be albino.